The effect of subthreshold prepulses on the recruitment order in a nerve trunk analyzed in a simple and a realistic volume conductor model |
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Authors: | Deurloo K E Holsheimer J Bergveld P |
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Institution: | (1) Department of Electrical Engineering, Institute for Biomedical Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, NL |
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Abstract: | The influence of subthreshold depolarizing prepulses on the threshold current-to-distance and the threshold current-to-diameter
relationship of myelinated nerve fibers has been investigated. A nerve fiber model was used in combination with both a simple,
homogeneous volume conductor model with a point source and a realistic, inhomogeneous volume conductor model of a monofascicular
nerve trunk surrounded by a cuff electrode. The models predict that a subthreshold depolarizing prepulse will desensitize
Ranvier nodes of fibers in the vicinity of the cathode and thus cause an increase in the threshold current of a subsequent
pulse to activate these fibers. If the increase in threshold current of the excited node is large enough, the excitation will
be accompanied by a strong hyperpolarization of adjacent nodes, preventing the propagation of action potentials in these fibers.
As fibers close to the electrode are more desensitized by prepulses than more distant ones, it is possible to stimulate distant
fibers without stimulating such fibers close to the electrode. Moreover, as larger fibers are more desensitized than smaller
ones, smaller fibers have lower threshold currents than larger fibers up to a certain distance from the electrode. The realistic
model has provided an additional condition for the application of this method to invert nerve fiber recruitment, i.e., real
or virtual anodes should be close to the cathode. When using a cuff electrode for this purpose, in the case of monopolar stimulation
the cuff length (determining the position of the virtual anodes) should not exceed twice the internodal length of the fibers
to be blocked. Similarly, the distance between cathode and anodes should not exceed the internodal length of these fibers
when stimulation is to be applied tripolarly.
Received: 15 May 2000 / Accepted in revised form: 9 February 2001 |
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